Continuous-wave-driven ultrasonic transducers have been used to measure velocity in changing flow patterns of flowing liquids by determining Doppler frequency shift in reflected ultrasonic pressure waves. Ultrasonic transducers of the type which have been used for this purpose are shown in FIGS. 1 and 2. As can be seen, they comprise electrically separated, solid piezoelectric material which serve as transmitters and receivers of ultrasound.
Commonly used piezoelectric materials, also useful in accordance with the present invention, are composites of piezo-ceramic and polymer. Typical of such materials are polymeric composites of lead-zirconate-titanate, lead-meta niobate and modified lead titanate.
To measure characteristics of flowing liquids, a high signal-to-noise ratio is required. Unfortunately, one of the major sources of noise in such devices is the cross coupling (also known as "cross talk") between transmitters and receivers. Excessive cross coupling results in reduced accuracy of measurements. This is especially undesirable where such devices are employed for medical applications since the erroneous results reduce diagnostic reliability.